When a document is converted into an electronic form so as to be stored, a scanner is often used as a unit for inputting an image. A scanner has a light source therein, and reads reflected light obtained by irradiating a document with light, by using a sensor such as a charge coupled device (CCD) so as to generate a digital image.
Since a condition such as a distance to the light source or the sensor is not varied, it is possible for a scanner to obtain a high-quality image. However, when the resolution of an image to be output is low, it is difficult to reproduce small characters clearly.
In addition, to reduce the data size of an obtained image, the obtained image is often subjected to, for example, an image compression process such as Joint Photographic Experts Group (JPEG). On the other hand, when lossy compression is used as an image compression process, fine characteristics of an image may be lost, thereby reducing the image quality.
Recently, there have been more opportunities to capture documents by using a digital camera so that document images are obtained. A document image is an image obtained by photographing a document including characters. When a document image is obtained using a digital camera, blurring in an image is likely to occur due to defocusing in comparison with a scanner. Hereinafter, image degradation due to various factors is called blurring of an image.
Character contours are unclear in a blurred image, thereby reducing readability of a document. In addition, there is a problem in that a blurred image leads to low accuracy in character recognition or the like performed to analyze the contents of a document image. Therefore, a technique for correcting blurring of an image so as to generate a clear image is desirable. Hereinafter, an operation of converting an image in which blurring occurs into a clearer image is called blur correction. An image in which blurring occurs is called a blurred image.
When the extent to which an image is blurred has already been determined, it is known that an inverse filter for correcting blurring or sharpening of an image enables an image to be restored to an image without blurring. Accordingly, a method is proposed in which the extent to which an input image is blurred (hereinafter, referred to as degree of blurring) is determined and the blurred image is appropriately corrected.
For example, a technique is proposed in which an additional device such as a distance sensor is used to determine the degree of blurring of an image. Another technique is proposed in which the degree of blurring is determined using a degree of sharpening of an edge which is obtained from an image of a human eye. Yet another technique is proposed in which the degree of blurring is determined by extracting a position of a character from an image and obtaining the width of pixels that are located in an edge region of the character and that are illustrated in halftones. These techniques are disclosed in, for example, Japanese Laid-open Patent Publication No. 2002-112099 and No. 2005-332382, and Japanese Patent No. 4315024.